Java BigInteger testBit 64位长

Question

This is probably a silly question, but I can't seem to find a simple answer.

I'm reading in a 64-bit (8 byte) long value and I'm then trying to use the BigInteger.testBit to see if bit 63 is set, as it's being used as a flag.

long value = 0x4000863; //This value is actually read from a file
Long.toBinaryString(value) = 100000000000000100001100011
BigInteger test = new BigInteger(Long.toString(value));
if (test.testBit(63)) {
    //yay
}
else {
    //boo
}

The above code is what I'm currently trying, and it says bit 63 is not set. As it's being stored as a long, I didn't think I'd have to pad the value, or am I just doing something wrong entirely?

Any input or advice would be greatly appreciated.

Thanks.

Answer 1

You are counting the bits wrong:

public void test() {
    // Binary - 100000000000000100001100011
    //          ^ This is bit 26
    long value = 0x4000863; 
    // Binary - ‭1000000000000000000000000000000000000000000000000000000000000000‬
    //          ^ THIS is bit 63
    long bigger = 0x8000000000000000L;
    BigInteger test = new BigInteger(Long.toString(value));
    System.out.println("L:" + Long.toBinaryString(value) + "\r\nB:" + test.toString(2) + "\r\nB63:" + test.testBit(63));
    test = new BigInteger(Long.toString(bigger));
    System.out.println("L:" + Long.toBinaryString(bigger) + "\r\nB:" + test.toString(2) + "\r\nB63:" + test.testBit(63));
}